If it were mine and there was only a 18" split from a vent to a wall, I would definitely try to repair it. If the vinyl is stapled around the vent, pull the staples and put some adhesive under the sheet next to the vent. I would also cut the sheet next to the wall and put a piece of trim over the cut. That will allow that future contraction can't pull at that area and cause another split.
FWIW, I replaced the vinyl floor with laminate, not unhappy about that decision at all. The laminate with attached rubber and an additional underlayment feels better under foot (but so do throw rugs). I think it also looks better.
Sound Guy's buddy has pretty good results and the only thing I can say is that every time they look at the floor they can marvel about the time spent installing it.